The invention relates to a power steering, in particular, to a steering force control system which controls an oil pressure acting upon a hydraulic reaction mechanism.
In a conventional power steering scheme which is responsive to a vehicle speed, a control valve is operated in accordance with an input applied to a steering wheel to introduce a pressure oil into one of chambers of a power cylinder to impart a steering assisting force to steerable road wheels while applying an oil pressure controlled by adding vehicle speed element to a hydraulic reaction chamber in a hydraulic reaction mechanism to produce a steering reaction, which may be sensed by a driver of the vehicle as a steering resistance.
FIG. 6 schematically shows an example of a conventional hydraulic reaction mechanism. A vehicle speed responsive pump 100 increases its discharge to a discharge passage 102 in response to an increase in the vehicle speed, and a variable throttle 104 is disposed in the passage. An oil pressure which is taken from the upstream side of the throttle 104 is introduced into a hydraulic reaction chamber 106 where a steering reaction is produced.
In a vehicle speed responsive power steering which is provided with a conventional hydraulic reaction mechanism as mentioned above, the pitch of controlling a reaction pressure steering force in a constant operative condition of a main control valve is determined by the following equation: ##EQU1## K: Constant Ratio Number .DELTA.P: Variable Pressure Rate of Hydraulic Reaction Chamber
where Q represents a discharge flow rate and A a cross sectional area of a throttle. Accordingly, there occurs a greater change toward the high speed end (see FIG. 7) which is undesirable in providing a proper steering sensation.